qubit-io 0.14.1

// =============================================================================
//    Copyright (c) 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================

use std::{
    hash::Hasher,
    io,
    pin::Pin,
    task::{
        Context,
        Poll,
    },
};

use crate::AsyncInput;

/// Asynchronous byte input that hashes successfully returned bytes.
///
/// Pending and failed reads do not change the hasher. The checksum algorithm
/// and stability guarantees are those of the supplied [`Hasher`].
///
/// # Type Parameters
///
/// - `I`: Wrapped asynchronous byte input type.
/// - `H`: Checksum hasher type.
#[must_use]
#[derive(Debug)]
pub struct AsyncChecksumInput<I, H> {
    /// Input whose successful reads are hashed.
    inner: I,
    /// Hasher tracking returned bytes.
    hasher: H,
}

impl<I, H> AsyncChecksumInput<I, H>
where
    H: Hasher,
{
    /// Creates a checksum-tracking asynchronous input.
    ///
    /// # Parameters
    ///
    /// - `inner`: Asynchronous byte input to wrap.
    /// - `hasher`: Hasher updated after successful reads.
    ///
    /// # Returns
    ///
    /// Returns an input with the supplied initial hasher state.
    #[inline(always)]
    pub const fn new(inner: I, hasher: H) -> Self {
        Self { inner, hasher }
    }

    /// Returns the current checksum value.
    ///
    /// # Returns
    ///
    /// Returns [`Hasher::finish`] for the current state.
    #[inline(always)]
    #[must_use]
    pub fn checksum(&self) -> u64 {
        self.hasher.finish()
    }

    /// Returns a shared reference to the wrapped input.
    ///
    /// # Returns
    ///
    /// Returns the wrapped asynchronous byte input.
    #[inline(always)]
    #[must_use]
    pub const fn inner(&self) -> &I {
        &self.inner
    }

    /// Returns a mutable reference to the wrapped input.
    ///
    /// Reads performed directly on the returned input do not update this
    /// wrapper's hasher.
    ///
    /// # Returns
    ///
    /// Returns the wrapped asynchronous byte input.
    #[inline(always)]
    #[must_use]
    pub fn inner_mut(&mut self) -> &mut I {
        &mut self.inner
    }

    /// Returns a shared reference to the hasher.
    ///
    /// # Returns
    ///
    /// Returns the current hasher state.
    #[inline(always)]
    #[must_use]
    pub const fn hasher(&self) -> &H {
        &self.hasher
    }

    /// Returns a mutable reference to the hasher.
    ///
    /// Mutating the returned hasher changes the checksum independently of
    /// bytes read through this wrapper.
    ///
    /// # Returns
    ///
    /// Returns the current hasher state mutably.
    #[inline(always)]
    #[must_use]
    pub fn hasher_mut(&mut self) -> &mut H {
        &mut self.hasher
    }

    /// Consumes this wrapper and returns both owned parts.
    ///
    /// # Returns
    ///
    /// Returns the wrapped input and final hasher state.
    #[inline(always)]
    #[must_use]
    pub fn into_parts(self) -> (I, H) {
        (self.inner, self.hasher)
    }
}

impl<I, H> AsyncInput for AsyncChecksumInput<I, H>
where
    I: AsyncInput<Item = u8>,
    H: Hasher,
{
    /// Byte item hashed after successful reads.
    type Item = u8;

    /// Preserves the wrapped input's buffering declaration.
    ///
    /// # Returns
    ///
    /// Returns the wrapped input's buffering declaration.
    #[inline(always)]
    fn is_buffered(&self) -> bool {
        self.inner.is_buffered()
    }

    /// Polls a read and hashes only bytes in a successful ready result.
    ///
    /// # Parameters
    ///
    /// - `cx`: Task context used to register a wake-up.
    /// - `output`: Destination byte slice.
    /// - `index`: Starting destination index.
    /// - `count`: Maximum number of bytes to read.
    ///
    /// # Returns
    ///
    /// Returns [`Poll::Pending`] when the input is not ready. A ready success
    /// contains the number of bytes read and hashed.
    ///
    /// # Errors
    ///
    /// Returns an I/O error reported by the wrapped input without changing the
    /// hasher.
    ///
    /// # Safety
    ///
    /// The range `index..index + count` must be valid for `output`.
    unsafe fn poll_read_unchecked(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        output: &mut [u8],
        index: usize,
        count: usize,
    ) -> Poll<io::Result<usize>> {
        // SAFETY: `inner` is never moved while projecting this pinned wrapper.
        let this = unsafe { self.as_mut().get_unchecked_mut() };
        let destination = &mut output[index..index + count];
        // SAFETY: The pinned wrapper keeps `inner` at a stable address.
        let inner = unsafe { Pin::new_unchecked(&mut this.inner) };
        match inner.poll_read(cx, destination) {
            Poll::Ready(Ok(read)) => {
                this.hasher.write(&output[index..index + read]);
                Poll::Ready(Ok(read))
            }
            Poll::Ready(Err(error)) => Poll::Ready(Err(error)),
            Poll::Pending => Poll::Pending,
        }
    }
}